I've been away from anything RF for almost 50 years, but I have both 2245A and 2247A scopes and have used the X-Y functions a little bit. I am very well old enough to be rusty at using something I haven't used in a long time, so I fully get where you are coming from.
I am confused by your statement about triggering on chan 3--if you are in XY mode, the scope's sweep generator is turned off and horizontal deflection is provided by the chan 1 input, so triggering is irrelevant--there is nothing to trigger.
Vertical deflection is provided by either chan 2, 3, or 4--I use chan 2 as you did because it has full functionality.
Turn off all of the chan 1-4 pushbuttons above the vertical attenuators--all lights in that row off. Ground the inputs of both chan 1 and chan 2 (both coupling selector lights off), and adjust intensity, chan 1 position, and horiz position for a dot in the center of the screen. Be sure to keep the intensity down so as not to burn your tube. Next, connect your X signal to the chan 1 input and choose AC coupling. Do not push the vertical button for chan1 as you would if making a voltage measurement. Adjust the chan 1 attenuator as necessary to get a horizontal line in the center of the screen. Again, it is important NOT to push the button to turn on channel 1 vertical--chan 1 is going to be your "horizontal sweep". Now connect your second (Y/vertical) signal to the chan 2 input, select AC coupling, and push the CHAN 2 button above the attenuators. Adjust the chan 2 attenuator as necessary. You should now see your X-Y display. Hope this helps!